A child is receiving intensive treatment for Legionnaires' disease believed to have been contracted during their stay at Starship Children's Hospital in Auckland.
Hospital director of health Dr Richard Aickin said that did not mean the health of other patients or staff were at risk.
The patient tested positive for Legionella but follow-up tests on nearby patients had returned negative results, he said.
There is no reason for the public to be alarmed, said Dr Aickin.
"Legionella does not spread from person-to-person and there is no reason for anyone who is or has been at Starship to be concerned.
"The usual incubation period has passed without a second case being identified, so staff are confident it is an isolated case."
Legionnaires' disease is an environmental organism usually originating from standing water that causes a chest infection.
It is the first case of Legionnaires' disease acquired by a patient at Starship or Auckland City Hospital, Dr Aickin said.